Alumina carrier particles

Fig. 7.33 Microscope pictures of alumina carrier particles with a metal precursor obtained under different drying conditions (F fast drying, S slow drying) (Desportes et al., 2005).

The key problem here is the preparation of these plates. The plates contained one inert hydrophobic part close to the hydrogen gas side, and another part consisting of a catalytically active metal on various types of carrier powder. The hydrophobic layer was made of 30-50-fjLm nonporous PTFE particles. The catalyst carrier particles were porous (mean pore diameter of 10 nm) with a particle size of about 5 p.m. The catalytic material was of three different types 10% Pd on alumina, 10% Pd on carbon, and 1.9% Pd on Ni0/Si02. In addition to these powder materials, the plates contained nets of nickel wire (0.16 mm) or glass fibers (0.2 mm) as reinforcement. The catalytic plates were prepared... 

When the rate is measured for a catalyst pellet and for small particles, and the diffusivity is also measured or predicted, it is possible to obtain both an experimental and a calculated result for rj. For example, for a first-order reaction Eq. (11-67) gives directly. Then the rate measured for the small particles can be used in Eq. (11-66) to obtain k. Provided is known, d) can be evaluated from Eq. (11-50) for a spherical pellet or from Eq. (11-56) for a fiat plate of.catalyst. Then 7caic is obtained from the proper curve in Fig. 11-7. Comparison of the experimental and calculated values is an overall measure of the accuracy of the rate data, effective diffusivity, and the assumption that the intrinsic rate of reaction (or catalyst activity) is the same for the pellet and the small particles. Example 11-8 illustrates the calculations and results for a flat-plate pellet of NiO catalyst, on an alumina carrier, used for the ortho-para-hydrogen conversion. 

The performance of the catalyst critically depends on the distribution of Co through the alumina carrier, as well as of Co particle size and distribution, degree of reduction, and pore structure of the alumina. The relationship between these parameters is not trivial, e.g. a high dispersion catalyst is not necessarily a good catalyst in terms of activity, selectivity and stability. Another aspect is the influence of impurities in raw materials, and during catalyst production and... 

In order to determine the dependence of the evaporation rate on solid particles with different composition and pore volume, we have used fine solid powders, e.g. SVC45 (a-alumina standard, total pore volume Vp = 0.037 cm /g), SVC50 (alumina carrier, Vp = 0.312 cm /g), SVC52 (kieselguhr, Vp = 0.009 cm /g) and SVC71 (graphite, Vp = 0.0608 cm /g). These powders and their data have been provided by Haldor Topsee A/S. 

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

Figure 4.1. Supported catalyst, consisting of small particles on a high surface area carrier such as silica or alumina, along with two simplified model systems, which in general offer much better opportunities for characterization at the molecular level.

For the same catalytically active material but with different catalyst carriers, different reaction rates and rate equations can be expected. Consider the hydrogenation of 2,4 DNT as discussed in Section 9.2 for 5% Pd on an active carbon catalyst with an average particle size of 30 (im [3]. These experiments were later repeated but with a Pd on an alumina catalyst [5]. This catalyst consisted of 4 x 4mm pellets, crushed to sizes of lower than 40/um in order to avoid pore diffusion limitations. In Figure 2.9 the measured conversion rates are given as a function of the averaged catalyst particle diameter, showing that above a diameter of 80/im the rate measured diminishes. For small particles they determined the rate equations under conditions where there were no pore diffusion lim-... 

The small number of charge carriers in the semiconductor is partially compensated for by the small size of metal particles. By using Schwab s magnetic measurements as a clue to the number of electrons transferred to the nickel across the nickel-alumina interface it can be estimated that a change of 0.05 electron per atom would produce detectable catalytic effects. By extrapolation to the small dispersed type of metal particle being considered here, it is seen that for a particle containing 2000 atoms the equivalent transfer would be produced by 100 electrons. Despite this, if one considers contact between a 2000-atom platinum particle and a 100 alumina particle (volume 4 x 10 cc) the flow of electrons to the metal would be drastically limited by the small number of charge... 

SAXS enables the size measurement of supported metallic particles. It is, however, necessary to eliminate the scanering due to the support. A comparison between a hydrated alumina and the same solid dehydrated (Fig. 10.23) shows the need to check in detail the scattering due to the interface formed via the pores of the carrier. The difference in asymptote for large values of Q is due to a difference in electron density contrast. It is only if the diagram of the support is recorded under the same conditions as those for the catalyst that it can be assumed that the difference is due solely to the particles. 

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